Thermoacoustic imaging (also called photoacoustic or optoacoustic imaging) is known for example from U.S. Pat. Nos. 5,840,023 and 5,713,356. Thermoacoustic imaging of tissue involves application of a short pulse of electromagnetic radiation to tissue and detection of acoustic waves emitted by absorbing structures within the tissue in response to the electromagnetic pulse. As used herein the term “acoustic” encompasses ultrasound, human audible sound, or infrasound and our combinations thereof; all these will also be referred to as sound. An electromagnetic radiation pulse causes abrupt heating and consequently localized temperature rise followed by thermal expansion at positions of radiation absorption, which results in generation of acoustic pulses from these positions. This is the photoacoustic effect.
The time of reception of the acoustic pulses at an acoustic detector, coupled acoustically to the tissue, is dependent on the distance between the sites of acoustic pulse generation and the detector. Thus acoustic detection at a single location can be used to determine the strength of the acoustic sources in the tissue at different distances. Detection of the acoustic pulses at a plurality of locations allows for the reconstruction in two- or three-dimensions of thermoacoustic images which are images of a measure of electromagnetic absorptivity of the tissue.
The reconstruction of such images requires knowledge of the speed of sound in the tissue to convert temporal resolution (or temporal phase resolution) into spatial resolution. In US patent application No. 2005/277834 a position independent speed of sound is used. Use of a position dependent speed of sound is described in an article titled “Correction of the effects of acoustic heterogeneity on thermoacoustic tomography using transmission ultrasound tomography” by Xin Jin, Lihong V. Wang and published in the 7th Conference on Biomedical Thermoacoustics, Optoacoustics and Acousto-optics, (Editor A. A. Oraevsky), Proceedings SPIE Vol 6086 6086W-32.
Wang proposes to use an additional measurement to the thermoacoustic experiment to measure the spatial variation of the speed of sound. This is an ultrasound transmission tomographic measurement. An ultrasound transmitter is used in addition to the acoustic receiver of the thermoacoustic experiment. From measurements of the arrival time of acoustic pulses generated by the transmitter through the sample in water, compared with the arrival time of acoustic pulses without the sample, the speed of sound is estimated. This measurement is performed taking projections around the sample. A speed of sound tomogram is generated using standard reconstruction concepts from x-ray computed tomography. The obtained spatial variations of the speed of sound are used to correct the thermoacoustic image instead of using a spatially independent single speed of sound. A disadvantage of the technique of Wang is that it requires a separate acoustic transmitter and a separate measurement.
From US2003/0167002 it is known to investigate the thermal effects of radiation on tissue by observing resulting changes in the speed of sound. However, this document does not concern thermoacoustic imaging.
Another acoustic transmission property that may affect the reliability of thermoacoustic images is position dependent attenuation of sound in the sample.